Abstract
BACKGROUND
As pain is a common symptom following a stroke, pressure pain threshold (PPT) assessment can be used to evaluate pain status or pain sensitivity of patients. However, the reliability of PPT test in stroke patients is still unknown.
AIM
To examine the intra- and inter-rater reliability of PPT measurements in poststroke survivors and explore their factors.
DESIGN
An observational study.
SETTING
The setting of the study is a rehabilitation hospital.
POPULATION
The population of the study was represented by a total of 54 patients after stroke.
METHODS
The study included 16 measured points on the affected and unaffected sides. PPT was assessed by two raters in turn. Intra- and inter-rater reliability was evaluated by intraclass correlation coefficients (ICC).
RESULTS
All intra-rater (ICC=0.84-0.97) and inter-rater (ICC=0.83-0.95) reliability for PPT assessment were good or excellent in stroke patients. Of the 16 points, 12 showed higher intra-rater ICC values than inter-rater, whereas no evident difference was observed between the affected and unaffected sides. Furthermore, patients who were male, ischemic, or with higher motor function generally performed higher ICC values than those who were female (24 out of 32 results), hemorrhagic (28 out of 32 results), or mobility dysfunction (26 out of 32 results), respectively.
CONCLUSIONS
PPT assessment with good or excellent reliability can be used in stroke patients. Neither of the two sides (affected or unaffected) affects PPT reliability, and intra-rater reliability is better than inter-rater reliability. In addition, gender, stroke type, and motor function can affect the reliability of measuring mechanical pain threshold in poststroke survivors.
CLINICAL REHABILITATION IMPACT
The pressure algometer can be used as a reliable and portable tool to assess the mechanical pain tolerance and sensory function in stroke patients in clinics.
Key words: Pain threshold, Stroke, Reproducibility of results
Stroke, the second cause of death worldwide, is a global health issue accompanied with high rates of disability and recurrence.1 One general problem after stroke is somatosensory impairment and the prevalence of sensory deficits in upper extremities is about 85%, regarding a decrease of touch, heat, cold and pain.2 On the other hand, poststroke pain is also a common symptom, and the prevalence varies from 19-74%.3 Pain after stroke can be divided into two types, which is central type caused by thalamic lesions, and peripheral type due to secondary impairments, abnormal muscle tone, or both.4 Therefore, previous studies have demonstrated that the pain sensory abnormalities after central nervous system injury include not only hyperalgesia but also hypoalgesia.5 As an important parameter of sensory function for pain, pressure pain threshold (PPT) and its reliability have been researched by several studies in healthy population and patients with different diseases, such as wrist fractures, piriformis syndrome, and patellofemoral pain.6-9 Meanwhile, some studies have investigated PPT in patients after central nervous system injury, such as stroke, multiple sclerosis, and Parkinson’s disease because sensory functions are commonly abnormal due to lesion of the central nervous system.10-13 The findings from these studies suggested that poststroke patient normally have wide range of PPT than healthy population, which means different individuals or affected muscles have different somatosensory abnormalities, such as hyperalgesia or hypoalgesia.12, 13 However, one study investigating the reliability of PPT assessment on neuropathic pain only including two poststroke participants.14 Another study only reported the test-retest reliability of PPT test on shoulder muscles in 23 poststroke patients.15 As can be seen, the intra-rater and inter-rater reliability of PPT measurement in stroke patients are still unclear. Furthermore, because most previous studies have focused on the comparison of reliability among individuals with or without pain,16, 17 other possible factors for the reliability of PPT are ignored. Our study aimed to explore the intra- and inter-rater reliability of PPT measurement in large muscles on the affected and unaffected sides in stroke patients. Furthermore, this study aimed to compare the reliability among different sides, raters, genders, stoke types, and motor function.
Materials and methods
Participants
Considering 3 repeated measurements in each participant with a significant level at 0.05, a power of 0.80, P0 (ICC)=0.5, and P1 (ICC)=0.8, the sample size should be not less than 14.4.18 Finally, 54 subjects were recruited from the first Rehabilitation Hospital of Shanghai and diagnosed with stroke by magnetic resonance imaging and other medical imaging methods. The inclusion criteria were as follows: 1) aged over 18; 2) stroke occurred 1 month to 10 years prior to baseline collection; and 3) could remain prone position until PPT tests were completed. The exclusion criteria were as follows: 1) stroke with bilateral side; 2) cannot respond to pressure pain properly or cannot understand the test content because of cognitive impairments; 3) trauma and other diseases may cause somatosensory abnormalities or pain; and 4) receiving any pain-relieving treatment within 2 weeks of the test. This study was approved by the Human Ethics Committee of the first Rehabilitation Hospital of Shanghai (YK-2020-01-030). Written consent was obtained from each subject prior to the experiment.
PPT measurement
Pressure pain threshold is defined as the minimal amount of pressure stimuli that allows subjects to feel pain transformed from pressure.19 The Wagner FDX-25 (Wagner Instruments, Greenwich, CT, USA), a handheld digital algometer, was used to measure PPT in this study. The unit used was kgf shorting for kgf/cm2, and the pressure algometer had a linear response to the applied force ranging from 0 kgf to 14 kgf, with a 1 cm2 round rubber tip. We used the following standardized instructions for PPT measurement: “I will put the rubber tip on your skin and generally exert pressure. If you feel a little unpleasant pain or discomfort but not pressure perception anymore, please tell me immediately and don’t need to endure.” “Unpleasant” was selected to describe sensation because it met the International Association for the Study of Pain definition of pain.20 During measurement, the 1 cm2 rubber disc of the algometer was placed perpendicular to the site marked by a black pen on the skin, and the pressure applied by the rater was slow and stable. All the subjects were asked to say “ok” as soon as the pressure perception transformed into pain. Then, the rater released the pressure and removed the rubber disc from the skin at once.
Study procedure
The baseline information about individual and stroke was collected, and the motor functions of participants were evaluated by Fugl-Meyer assessment. Researchers interpreted the test procedure for participants and required them to practice the assessment on the unaffected forearm muscle 3 to 8 times. Patients who did not understand or complete the test procedure correctly after eight-time practice were excluded from this study. Then, sixteen measured sites were marked on skin at both sides (8 sites at each side) prior to formal assessment, which were defined as muscle bellies of the 1) middle deltoid – midpoint of the line between the humeral deltoid trochanter and the acromion; 2) biceps brachii: 9 cm above the crease on the elbow; 3) rectus femoris: midpoint of the line between the anterior inferior iliac spine and the upper margin of the patella; 4) tibialis anterior: 5 cm below and 2.5 cm lateral to the tibial tubercle; 5) erector spinae at the L2 level: the level of the second lumbar vertebrae (L2) and 2 cm from the midline; 6) erector spinae at the L4 level: the level of the fourth lumbar vertebrae (L4) and 2 cm from the midline; 7) biceps femoris: the midpoint of the line between the ischium tubercle and popliteal fossa; and 8) medial gastrocnemius: 30% of the line from the popliteal fossa to the lateral malleolus.21 Considering data accuracy and measurement convenience, all subjects were asked to maintain a supine position during the PPT measurement at the first four sites and a prone position at the last four sites. For the inter-rater reliability, rater A assessed subjects firstly (test 1) followed by rater B. For the intra-rater reliability, rater A repeated the same test procedure (test 2) immediately after rater B. The test procedure required rater to test each site twice with an interval of 20 s to 40 s, and all sites at the unaffected side were tested preferentially. In other words, each site was measured six times in three test procedures executed by rater A in test 1, rater B, and rater A in test 2 in turn. The rater order was immutable in all subjects.
Statistical analysis
All statistical analyses were processed by IBM SPSS Statistics 25.0 (SPSS Inc., Chicago, IL, USA). The intra-rater reliability (tested by rater A in test 1 and 2) and inter-rater reliability (tested by rater A and rater B) of the PPT measurement were determined by calculating intraclass correlation coefficients (ICC) with 95% confidence intervals (CI) in a two-way mixed effect model. Standard error of measurement (SEM) and minimum detectable change (MDC) were calculated using the following formula: SEM = standard deviation×(1-ICC)1/2, MDC=1.96×SEM×21/2. The systematic error and level of agreement in intra- and inter-reliability were indicated through Bland-Altman plots. We divided all participants into two subgroups according to gender and stroke type, respectively. According to the median value of motor function assessed by the Fugl-Meyer, all participants were also divided into two subgroups, which were below-median and above-median subgroups. Then, the ICC of intra- and inter-rater reliability among the male and female, hemorrhagic and ischemic, or below-median and above-median subgroups was calculated to evaluate the factors for PPT reliability.
Results
Demographic and characteristics
The study enrolled 54 stroke patients who were all right-handed. The baseline characteristics of the subjects are presented in Table I. Most of the subjects were elderly (the mean age was 65.52±11.57 years), and the mean BMI was 23.65±3.28 kg/m2. The percentage of female patients was 35.2% and of patients with right affected side were 51.9%. Moreover, hemorrhagic stroke accounted for 31.5%, whereas ischemic stroke accounted for 68.5%.
Table I. —Demographics and characteristics of the participants.
| Variables | All participants | Gender subgroups | Stroke type subgroups | ||
|---|---|---|---|---|---|
| (N.=54) | Male (N.=35) | Female (N.=19) | Hemorrhage (N.=17) | Ischemia (N.=37) | |
| Age (years) | 65.52±11.57 | 63.43±10.86 | 69.37±12.14 | 60.18±10.75 | 67.97±11.23 |
| BMI (kg/m2) | 23.65±3.28 | 24.09±2.51 | 22.85±4.32 | 24.38±3.50 | 23.32±3.16 |
| Smoking, N. (%) | 23 (42.6%) | 22 (62.9%) | 1 (5.3%) | 10 (58.8%) | 13 (35.1%) |
| Drinking, N. (%) | 17 (31.5%) | 16 (45.7%) | 1 (5.3%) | 9 (52.9%) | 8 (21.6%) |
| Right dominant hand, N. (%) | 54 (100.0%) | 35 (100.0%) | 19 (100%) | 17 (100%) | 37 (100%) |
| Right affected side, N. (%) | 28 (51.9%) | 18 (51.4%) | 10 (52.6%) | 10 (58.8%) | 18 (48.6%) |
| Stroke onset (days) | 426.09±456.02 | 468.80±541.41 | 347.42±220.31 | 558.82±384.29 | 365.11±477.89 |
| FMA-MF | 46.41±27.16 | 47.97±27.82 | 44.05±25.86 | 34.47±26.74 | 52.16±25.53 |
| FMA-JP | 35.52±9.85 | 35.49±9.98 | 35.58±9.86 | 35.41±9.52 | 35.57±10.12 |
| Barthel Index | 63.30±21.67 | 63.69±21.53 | 62.58±22.51 | 59.65±23.12 | 64.97±21.09 |
Data are presented as mean±standard deviation or as number of participants (%). BMI: Body Mass Index; FAM: Fugl-Meyer Assessment; MF: motor function; JP: joint pain.
ICC values and reliability
The ICC values were interpreted as follows: 0.60<ICC<0.75, moderate; 0.75<ICC<0.9, good; and ICC>0.9, excellent reliability.22 The ICC, SEM, and MDC values of intra-rater and inter-rater reliability in each measured muscle are shown in Table II.
Table II. —Intra- and inter-rater reliabilities in eight measured points at the affected and unaffected sides.
| ICC (95% CI) | SEM (kgf) | MDC (kgf) | ||
|---|---|---|---|---|
| Middle deltoid | ||||
| UA | Intra-rater | 0.87 (0.76-0.93) | 0.55 | 1.54 |
| Inter-rater | 0.90 (0.82-0.94) | 0.49 | 1.36 | |
| A | Intra-rater | 0.89 (0.75-0.94) | 0.46 | 1.29 |
| Inter-rater | 0.88 (0.74-0.94) | 0.45 | 1.26 | |
| Biceps brachii | ||||
| UA | Intra-rater | 0.84 (0.51-0.93) | 0.32 | 0.88 |
| Inter-rater | 0.84 (0.73-0.91) | 0.33 | 0.91 | |
| A | Intra-rater | 0.85 (0.62-0.93) | 0.40 | 1.10 |
| Inter-rater | 0.83 (0.70-0.90) | 0.37 | 1.03 | |
| Rectus femoris | ||||
| UA | Intra-rater | 0.93 (0.84-0.96) | 0.45 | 1.25 |
| Inter-rater | 0.93 (0.88-0.96) | 0.46 | 1.27 | |
| A | Intra-rater | 0.91 (0.77-0.95) | 0.44 | 1.23 |
| Inter-rater | 0.88 (0.70-0.95) | 0.52 | 1.44 | |
| Tibialis anterior | ||||
| UA | Intra-rater | 0.92 (0.86-0.96) | 0.59 | 1.64 |
| Inter-rater | 0.90 (0.81-0.95) | 0.71 | 1.96 | |
| A | Intra-rater | 0.86 (0.72-0.93) | 0.80 | 2.20 |
| Inter-rater | 0.87 (0.74-0.93) | 0.78 | 2.17 | |
| Erector spinae (L2) | ||||
| UA | Intra-rater | 0.96 (0.93-0.98) | 0.59 | 1.64 |
| Inter-rater | 0.92 (0.87-0.95) | 0.82 | 2.28 | |
| A | Intra-rater | 0.96 (0.93-0.98) | 0.62 | 1.73 |
| Inter-rater | 0.94 (0.90-0.97) | 0.76 | 2.10 | |
| Erector spinae (L4) | ||||
| UA | Intra-rater | 0.96 (0.94-0.98) | 0.58 | 1.61 |
| Inter-rater | 0.95 (0.92-0.97) | 0.64 | 1.77 | |
| A | Intra-rater | 0.97 (0.95-0.98) | 0.54 | 1.50 |
| Inter-rater | 0.95 (0.92-0.97) | 0.66 | 1.82 | |
| Biceps femoris | ||||
| UA | Intra-rater | 0.91 (0.82-0.95) | 0.65 | 1.81 |
| Inter-rater | 0.85 (0.42-0.94) | 0.80 | 2.23 | |
| A | Intra-rater | 0.93 (0.89-0.96) | 0.61 | 1.69 |
| Inter-rater | 0.90 (0.82-0.94) | 0.68 | 1.87 | |
| Medial gastrocnemius | ||||
| UA | Intra-rater | 0.91 (0.75-0.96) | 0.46 | 1.28 |
| Inter-rater | 0.85 (0.67-0.92) | 0.60 | 1.65 | |
| A | Intra-rater | 0.90 (0.82-0.94) | 0.57 | 1.59 |
| Inter-rater | 0.85 (0.57-0.93) | 0.70 | 1.94 | |
MDCs are at the 95% confidence level. UA: unaffected side; A: affected side; ICC: intraclass correlation coefficients; CI: confidence interval; SEM: standard error of measurement; MDC: minimal detectable change; L2: the second lumbar vertebrae; L4: the fourth lumbar vertebrae.
All intra- and inter-rater reliability of PPT test in bilateral muscles in stroke patients were good or excellent (ICC>0.8). The ICC values of the intra-rater reliability ranged from 0.84 to 0.97, and those of the inter-rater reliability ranged from 0.83 to 0.95. Regarding intra-rater reliability, the highest ICC (0.97) was observed in the erector spinae at the L4 level on the affected side, whereas the lowest ICC (0.84) was found in the biceps brachii on the unaffected side. As for inter-rater reliability, the highest ICC value (0.95) was observed in the erector spinae at the L4 level on both sides, whereas the lowest ICC value (0.83) was found in the biceps brachii on the affected side. Furthermore, the erector spinae showed better reliability at the L2 or L4 levels (ICC>0.9 on the bilateral side). By contrast, the biceps brachii showed relatively lower reliability (ICC ≤ 0.85 on the bilateral side) than the other muscles in this study. No significant superiority of neither the intra-rater reliability nor inter-rater reliability was found between the affected side and unaffected side. ICCs for the intra-rater reliability were better than those for the inter-rater reliability with higher values in 12 out of 16 measured sites in this study. From the perspective of consistency of agreement, the maximal mean difference in intra-rater reliability was observed in the affected tibialis anterior (0.57 kgf; 95% limits of agreement [LOAs], -1.66 kgf to 2.79 kgf; Figure 1B).
Figure 1.
—Bland-Altman plots for reliability in tibialis anterior muscle: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) muscle at the unaffected side; and B, D) muscle at the affected side.
The minimal mean difference was found in the affected erector spinae muscle at the L2 level (-0.08 kgf; 95% LOAs, -1.74 kgf to 1.57 kgf; Figure 2B). As for inter-rater reliability plots, the maximal and minimal mean differences were observed in the unaffected biceps femoris (0.84 kgf; 95% LOAs, -0.96 kgf to 2.63 kgf; Figure 3C) and the affected erector spinae at the L2 level (-0.01 kgf; 95% LOAs, -2.02 kgf to 2.00 kgf; Figure 2D), respectively. Furthermore, the mean differences in erector spinae at the L2 and L4 levels were almost zero. More Bland–Altman plots about other muscles can be found in Supplementary Digital Material 1 (Supplementary Figure 1-5).
Figure 2.
—Bland-Altman plots for reliability in erector spinae muscle at the L2 level: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) L2 level of muscle at the unaffected side; and B, D) L2 level of muscle at the affected side.
Figure 3.
—Bland-Altman plots for reliability in biceps femoris muscle: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) muscle at the unaffected side; and B, D) muscle at the affected side.
Reliabilities in gender, stroke type and motor function subgroups
The ICC values of reliability in different gender, stroke type and motor function subgroups are presented in Table III.
Table III. —The ICC values for PPT measurement in different gender, stroke type, and motor function subgroups.
| Gender (N.=54) | Stroke type (N.=54) | Motor function (N.=54) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Female (N.=19) | Male (N.=35) | Hemorrhagic (N.=17) | Ischemic (N.=37) | Below-median (N.=26) | Above-median (N.=28) | |||||||
| Intra-rater ICC | Inter-rater ICC | Intra-rater ICC | Inter-rater ICC | Intra-rater ICC | Inter-rater ICC | Intra-rater ICC | Inter-rater ICC | Intra-rater ICC | Inter-rater ICC | Intra-rater ICC | Inter-rater ICC | |
| MD-UA | 0.83 | 0.88 | 0.85* | 0.88 | 0.79 | 0.82 | 0.93* | 0.96* | 0.84 | 0.87 | 0.90* | 0.93* |
| MD-A | 0.72 | 0.82 | 0.88* | 0.86* | 0.92 | 0.91 | 0.87 | 0.87 | 0.93 | 0.89 | 0.87 | 0.87 |
| BB-UA | 0.79 | 0.76 | 0.82* | 0.82* | 0.86 | 0.82 | 0.83 | 0.86* | 0.84 | 0.85 | 0.86* | 0.84 |
| BB-A | 0.64 | 0.66 | 0.84* | 0.82* | 0.75 | 0.75 | 0.92* | 0.90* | 0.79 | 0.78 | 0.90* | 0.87* |
| RF-UA | 0.94 | 0.91 | 0.90 | 0.91 | 0.92 | 0.91 | 0.93* | 0.94* | 0.93 | 0.92 | 0.93 | 0.93* |
| RF-A | 0.82 | 0.78 | 0.92* | 0.90* | 0.93 | 0.88 | 0.90 | 0.89* | 0.85 | 0.82 | 0.94* | 0.93* |
| TA-UA | 0.84 | 0.75 | 0.94* | 0.94* | 0.89 | 0.87 | 0.93* | 0.91* | 0.88 | 0.82 | 0.94* | 0.93* |
| TA-A | 0.82 | 0.71 | 0.85* | 0.90* | 0.73 | 0.77 | 0.89* | 0.89* | 0.65 | 0.76 | 0.94* | 0.92* |
| L2-UA | 0.97 | 0.91 | 0.95 | 0.91 | 0.91 | 0.84 | 0.97* | 0.94* | 0.91 | 0.84 | 0.98* | 0.94* |
| L2-A | 0.96 | 0.93 | 0.96 | 0.94* | 0.92 | 0.93 | 0.97* | 0.95* | 0.91 | 0.90 | 0.98* | 0.96* |
| L4-UA | 0.95 | 0.94 | 0.96* | 0.95* | 0.94 | 0.92 | 0.97* | 0.96* | 0.94 | 0.90 | 0.97* | 0.97* |
| L4-A | 0.95 | 0.94 | 0.97* | 0.95* | 0.94 | 0.93 | 0.98* | 0.96* | 0.94 | 0.92 | 0.98* | 0.97* |
| BF-UA | 0.90 | 0.85 | 0.91* | 0.84 | 0.89 | 0.78 | 0.92* | 0.87* | 0.92 | 0.84 | 0.91 | 0.87* |
| BF-A | 0.94 | 0.85 | 0.92 | 0.90* | 0.84 | 0.80 | 0.96* | 0.93* | 0.91 | 0.87 | 0.95* | 0.93* |
| MG-UA | 0.85 | 0.78 | 0.92* | 0.86* | 0.87 | 0.82 | 0.92* | 0.86* | 0.90 | 0.86 | 0.91* | 0.85 |
| MG-A | 0.82 | 0.81 | 0.92* | 0.86* | 0.86 | 0.76 | 0.91* | 0.87* | 0.80 | 0.69 | 0.94* | 0.92* |
UA: unaffected side; A: affected side; MD: middle deltoid muscle; BB: biceps brachii muscle; RF: rectus femoris muscle; TA: tibialis anterior muscle; L2 and L4: erector spinae muscles at L2 and L4 levels; BF: biceps femoris muscle; MG: medial gastrocnemius muscle; ICC: intraclass correlation coefficients. *ICC value in the subgroup is higher than that in another subgroup.
The male and female subgroups had 35 and 19 participants, respectively. The ICC values in the male subgroup ranged from 0.82 to 0.97, whereas those in the female subgroup ranged from 0.64 to 0.97. Meanwhile, 24 out of 32 ICC values for intra- or inter-rater reliability were higher in the male than female subgroup. Regarding the stroke-type subgroups, ischemic and hemorrhagic subgroups had 37 and 17 participants, respectively. The ICC values in the ischemic subgroup ranged from 0.83 to 0.98, whereas those in the hemorrhagic subgroup ranged from 0.73 to 0.94. The ischemic subgroup showed greater ICC than hemorrhagic participants in 28 out of 32 values. According to the motor function, there were 26 and 28 participants in below- and above-median subgroups, respectively. The ranges of ICC values were 0.65-0.94 in below-median subgroup and 0.84–0.98 in above-median subgroup. The above-median subgroup had a higher ICC than the below-median subgroup in 26 out of 32 values.
Discussion
This study evaluated the reliability of PPT assessment in stroke patients. The results showed that the intra- and inter-rater reliability were all good or excellent (ICC>0.8) in all measured muscles, particularly muscles in the low back region. While no significant difference in PPT reliability was found between affected and unaffected side, the intra-rater reliability was superior to inter-rater reliability in most measured muscle. In addition, poststroke patients who were male, ischemic or with higher functional mobility generally showed better reliability than who were female, hemorrhagic, or with lower motor function. All ICC values in this study indicated that PPT assessment was reliable in stroke patients. Moreover, intra-, and inter-rater reliability were not affected by the hemiparetic side. Therefore, it is reliable to apply the PPT assessment among poststroke survivors in clinics on the not only affected but also unaffected side. Similarly, the great reliability of PPT measurement has also been demonstrated in other muscles and other diseases. For example, the PPT test showed good or excellent intra-, inter-rater, and test-retest reliability when measuring shoulder muscles in patients with and without unilateral subacromial impingement syndrome (ICC=0.87-0.98).23 Furthermore, almost perfect intra-rater reliability (ICC=0.82–0.94) and within-session test–retest reliability (ICC=0.85-0.91) were found during assessing PPT in people with dizziness caused by neck diseases.24 This study provides new evidence for the high reliability of PPT measurement in stroke patients. The ICC values, and mean differences obtained by Bland-Altman plots suggested that the muscles in the low back region whether at the L2 or L4 level showed better reliability than the other muscles. In addition, the worst intra- and inter-rater reliability were detected in the biceps brachii, which was similar to a study reporting that mechanical pain threshold of muscles in the shoulder region is more labile than the other muscles among the healthy population.25 Furthermore, two studies pointed out the inter-rater ICCs for the tibialis anterior were 0.91 and 0.90, respectively.26, 27 Consistent with the aforementioned findings, this study reported an intra-rater ICC at 0.90 and inter-rater ICC at 0.92 on the unaffected side. These phenomena might indicate that the variation of pain threshold in low back muscles was slight, whereas the biceps brachii, commonly affected by hemiplegia, showed a more variable pain threshold than others. Moreover, the intra-rater ICCs were generally higher than the inter-rater ICCs. This finding has been demonstrated in a previous study that reported the almost perfect intra-rater reliability (ICC=0.94-0.97) and near perfect inter-rater reliability (ICC=0.79-0.90) of PPT assessment in healthy people and patients with neck pain.26 This phenomenon was commonly found in clinical assessments because of the more different details during between-operator than within-operator measurement. In this study, the difference between intra- and inter-rater reliability could be caused by the different raters produce various directions and velocities of pressure stimuli application, and different reaction duration when assessing PPT. Notably, although the inter-rater reliability was worse than the intra-rater reliability in the present study, all reliabilities in measured muscles were good or excellent regardless of healthy or hemiparetic side. Except for operator and muscle, the study also determined other factors, gender, stroke type, and motor function, for the reliability of PPT assessment. The first important factor was gender, indicating that female patients showed worse reliability than male in most measured muscles. Similarly, a study reported that healthy women showed significantly lower ICCs than men when assessing mechanical detection and pain thresholds.28 This gender difference could be caused by some biological, psychological, and sociological differences between female and male patients. Therefore, the complex interaction of endogenous pain modulatory capacity, sex hormones, depression conditions after stroke, and roles in the family and society could contribute to the low reliability of PPT assessment in female individuals.29-31 Meanwhile, another factor for reliability of PPT test was stroke type, which means a better reliability on most muscles in ischemic stroke than hemorrhagic stroke. This specific finding in stroke patients was first reported by this study. The reliability difference caused by stroke type might be associated with various somatosensory impairments caused by different brain lesions.32, 33 However, further research is required to explore the potential mechanisms of the effects of brain lesions after a stroke on pain threshold and their range of variation. Furthermore, patients with better motor function generally have a greater reliability of PPT assessment than others, either on the affected or unaffected side. One possible reason for this finding could be a significant relationship between motor and cognitive functions in poststroke patients,34 which means less functional mobility is associated with worse cognition, thereby affecting the accuracy of PPT assessment. Secondly, motor dysfunction after stroke is normally caused by severe spasticity that relies on the severity of brain lesion.35 According to a previous study, the stroke severity is an independent factor for somatosensory impairment, which might indicate that serious motor dysfunction is associated with a variable PPT.36 However, since the brain lesion of stroke and poststroke symptoms are complicated and multifactorial, it is hard to determine the exact reason for this finding. Notably, MDCs varied from 0.88 kgf to 2.28 kgf in muscles on the unaffected side and ranged from 1.03 kgf to 2.63 kgf on the affected side. This finding indicated that the assessment might be not accurate when being applied to measure the change of PPT in patients with a low pain threshold. In other words, if a patient is sensitive to pressure pain and the initial PPT is lower than MDC, then the assessment cannot accurately detect the reduction of PPT in case the hyperalgesia, a common symptom after a stroke, is present.12, 37 In addition to the pressure algometer used in the present study, the reliability of other devices used to assess PPT has been explored. For instance, the PPT assessment by a digital algometry showed good inter-rater reliability (ICC=0.64–0.92) and test–retest reliability (ICC=0.72-0.95) on unaffected and affected piriformis muscles in patients with piriformis syndrome.8 Moreover, a study reported that a dynamometer with a 1 cm2 metal tip was used to assess PPT in patients with patellofemoral pain syndrome, and the inter-rater reliability of PPT was good (ICC=0.79).9 The slight difference in ICC values from previous studies might be caused by different device, operators, and participants. Overall, the reliability of PPT measurement was generally good regardless of the device or population. This study is the first to evaluate intra- and inter-rater reliability of PPT test in poststroke survivors. Furthermore, this study included the affected and unaffected sides in large muscles all over the body. In addition, the intra- and inter-rater reliability in different gender, stroke type and motor function subgroups were also evaluated to explore factors for the reliability.
Limitations of the study
This study has also some limitations. First, the inclusion criteria in this study were not focused on some specific conditions, which led to difference in the number of participants between different gender and stoke type subgroups. Whether the number difference affected the ICC values or not during between-subgroup comparison remained unknown. Second, considering that different region of brain lesion could affect somatosensory functions,32, 38 more detailed classifications of stroke or neuroimaging, not covered in this study, could provide more information regarding the factors affecting intra- and inter-rater reliability. Furthermore, the participants recruited generally had proper cognitive functions and psychological conditions to ensure accurate response to pressure pain, which also indicated the disadvantage of this PPT assessment in patients with serious cognitive dysfunctions.
Conclusions
In this study, a total of 16 points on the unaffected and affected sides were selected as measurement points to explore the intra-rater and inter-rater reliability of PPT assessment in stroke patients. The results indicate that all intra- and inter-rater reliability is good or excellent. Moreover, the measured muscles, rater, gender, stoke type, and motor function rather than the body side in stroke patients can affect the reliability. The pressure algometer can be used as a reliable and portable tool to assess the mechanical pain tolerance and sensory function in stroke patients in clinics. Further studies with larger sample size, stricter inclusion and exclusion criteria, and more quantitative measurement for related brain mechanisms are required to obtain more detail information about mechanical pain thresholds in poststroke patients.
Supplementary Digital Material 1
Supplementary Figure 1
Bland-Altman plots for reliability in middle deltoid muscle: A, B) Intra-rater reliability; C, D) inter-rater reliability; A, C) Muscle at the unaffected side; B, D) muscle at the affected side.
Supplementary Figure 2
Bland-Altman plots for reliability in biceps brachii muscle: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) muscle at the unaffected side; B, D) muscle at the affected side.
Supplementary Figure 3
Bland-Altman plots for reliability in rectus femoris muscle: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) muscle at the unaffected side; B, D) muscle at the affected side.
Supplementary Figure 4
Bland-Altman plots for reliability in erector spinae muscle at the L4 level: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) the L4 level of muscle at the unaffected side; B, D) L4 level of muscle at the affected side.
Supplementary Figure 5
Bland-Altman plots for reliability in medial gastrocnemius muscle: A, B) intra-rater reliability; C, D) inter-rater reliability. A, C) muscle at the unaffected side; B, D) muscle at the affected side.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Supplementary Figure 1
Bland-Altman plots for reliability in middle deltoid muscle: A, B) Intra-rater reliability; C, D) inter-rater reliability; A, C) Muscle at the unaffected side; B, D) muscle at the affected side.
Supplementary Figure 2
Bland-Altman plots for reliability in biceps brachii muscle: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) muscle at the unaffected side; B, D) muscle at the affected side.
Supplementary Figure 3
Bland-Altman plots for reliability in rectus femoris muscle: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) muscle at the unaffected side; B, D) muscle at the affected side.
Supplementary Figure 4
Bland-Altman plots for reliability in erector spinae muscle at the L4 level: A, B) intra-rater reliability; C, D) inter-rater reliability; A, C) the L4 level of muscle at the unaffected side; B, D) L4 level of muscle at the affected side.
Supplementary Figure 5
Bland-Altman plots for reliability in medial gastrocnemius muscle: A, B) intra-rater reliability; C, D) inter-rater reliability. A, C) muscle at the unaffected side; B, D) muscle at the affected side.